Since the discovery of advantages of phosphate base laser glass over silicate base glass, several neodymium doped laser glass compositions with a phosphate matrix have been reported in various patents such as U.S. Pat. Nos. 4,075,120, 4,248,732, 4,239,645, and 4,333,848. The general emphasis of those glasses on high stimulated emission cross section, good optical quality and chemical durability have resulted in laser glasses beneficial to the purpose of generating high peak power output.
The laser glasses of U.S. Pat. No. 4,075,120 are generically described as comprising, in mole percent 35.0 to 65.0% P.sub.2 O5; 0.01 to 15.0% R.sub.2 O.sub.3 (Al.sub.2 O.sub.3, La.sub.2 O.sub.3, Y.sub.2 O.sub.3, Tm.sub.2 O.sub.3, B.sub.2 O.sub.3, Er.sub.2 O.sub.3 and mixtures thereof); 5.0 to 30.0% RO (alkali earth metal oxides. disclosing BaO, BeO, MgO, SrO, CaO and mixtures thereof): 5.0 to 40.0% R.sub.2 O (alkali metal oxides including Li.sub.2 O, K.sub.2 O, Na.sub.2 O, Rb.sub.2 O and mixtures thereof) and 0.01 to 7.0% of an oxide of a trivalent rare earth ion, (Nd.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Pm.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Er.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Tb.sub.2 O.sub.3). In the specific glass formulations disclosed in this patent. Li.sub.2 O when present is used in a very large amount (minimum of 25.0 mol % in Example XIII, while Al.sub.2 O.sub.3 when present is used in 5 mol % or less. Naturally, the high Li.sub.2 O content necessarily reduces P.sub.2 O.sub.5 maximum, with a high up 55 mol % P.sub.2 O.sub.5 used when Li.sub.2 O and Al.sub.2 O.sub.3 are in the same glass.
U.S. Pat. No. 4,248,732 is a continuation-in-part of U.S. Pat. No. 4,075,120. ZnO is added as another RO possibility. Example XXVIII includes lowered Li.sub.2 O content (15.0 mol %), but also reduced P.sub.2 O.sub.5 (50.0 mol. %) due to high ZnO content (30.0 mol %)
U.S. Pat. No. 4,333,848 generally describes laser glasses comprising in mole percent, 55 to 70% P.sub.2 O.sub.5 ; 3 to 15% alkali metal oxide (preferably Li.sub.2 O plus K.sub.2 O with 6 to 12% Li.sub.2 O stated to be a preferred embodiment); 10 to 30% BaO, 0 to 15% CaO and 0 to 15% SrO with total RO concentration of 20 to 28%; 0.5 to 5% Al.sub.2 O.sub.3 ; 0.5 to 11% Nd.sub.2 O.sub.3 and 1 to 5% solarization inhibitor (stated to be Sb.sub.2 O.sub.3, Nb.sub.2 O.sub.3 and SiO.sub.2). The specific glass formulating examples of this patent, when including Li.sub.2 O within the preferred range, employ relatively low levels of Al.sub.2 O.sub.3 (say 2.5%) and only BaO and CaO as RO components.
U.S. Pat. No. 4,239,645, assigned to one of the assignees hereof, generically describes laser glasses comprising, in mole percent, 55 to 70% P.sub.2 O.sub.5 1-15% Al.sub.2 O.sub.3 (preferably 4-10); 10-25% Li.sub.2 O, Na.sub.2 O and/or K.sub.2 O (preferably 9-15% Li.sub.2 O, 4-10% Na.sub.2 O); 5-15% BaO. ZnO, CaO, SrO and/or MgO (preferably 5-12% of total of CaO, MgO, CaF.sub.2 and MgF.sub.2); 0.01-5% Nd.sub.2 O.sub.3, and various optional oxides. This patent includes sixty-three examples, most of which include alkali metal oxide, especially Na.sub.2 O in large amounts, and without regulation of alkaline earth metal oxide content by amount and selection of oxide.
For the purpose of some laser systems such as Inertial Confinement Fusion (ICF) which investigates the fusion of hydrogen isotopes to helium, the above mentioned glasses have been found to be quite useful because an extremely high peak power is required for this fusion reaction. The pulse repetition frequency is very low for this application, usually only one pulse per several hours. Thermal shock resistance of the glass developed for this application (ICF) does not need to be and is not very high.
For more practical applications, the pulse repetition frequency is expected to be much higher than for the ICF application. A high average power output instead of high peak power output becomes the top priority. In order to generate high average power output, the lasing substance must be able to operate at high pulse repetition frequency at a relatively high pulse energy. The maximum pulse repetition frequency f for a slab laser can be determined by ##EQU1## wherein R is the so-called thermal shock resistance parameter, t is the thickness, p is the peak stored energy density, and x is the ratio of thermal energy deposited in the material to peak optical energy stored in the material. This indicates that the maximum pulse repetition frequency depends on a thermal shock resistance parameter R.
The thermal shock resistance parameter R is normally evaluated by the equation of ##EQU2## wherein S is the maximum permissible surface tensile stress, K is the thermal conductivity, V is Poisson ratio, .alpha. is the linear thermal expansion coefficient and E is the Young's modulus. For brittle materials such as glass, the maximum tensile stress S is an extrinsic property that depends more strongly on the type and size of surface defects than on the intrinsic strength. For comparison of one material with another a more appropriate intrinsic fracture toughness K.sub.IC is used instead of S (Marion, J. E., J. Appl. Phys. 62(5), 1595 (1987)). The intrinsic thermal shock resistance parameter R' can be represented by the equation of ##EQU3##
This invention relates to the composition of a phosphate based laser glass with strong thermal shock resistance for use in high average power applications. Glasses of the present invention have R' values of 0.9-1.02 W/m.sup.1/2. Representative phosphate base laser glasses described in the preceding patents have R' value around 0.5-0.7 W/m.sup.1/2, while a more desirable R' value for glasses for use in high average power applications is at least 0.9 W/m.sup.1/2.
For the purpose of high average power output application, any platinum particle inclusion from the melting crucible tends to cause a thermal fracture. Laser glass with compositions in the silicate or silica-phosphate system usually has a higher R' value over phosphate glass. Unfortunately, those glasses have been found to be too difficult to manufacture without any platinum inclusion because of their slow dissolution rate of platinum. Such is the problem that has existed with an earlier glass of Japanese Patent Application No. 60-53424, published as 61-215233. The present glass is based on a phosphate matrix with high platinum solubility and therefore may be manufactured without such inclusions.
In general laser material should have a high gain coefficient in order to obtain a high laser output efficiency. The gain coefficient g of a laser medium is related to the stimulated emission cross section .sigma. as in the following equation EQU g=(W.sub.p N.sub.t T.sub.f).sigma.
wherein W.sub.p is the pumping rate, N.sub.t is the ionic concentration of lasing element, T.sub.f is the fluorescence lifetime. It indicates that the gain coefficient is proportional to the stimulated emission cross section. Glasses that have cross sections of at least 3.4.times.10.sup.-20 cm.sup.2 at 1.05 .mu.m, are desired for normal high average power application.